1. Field of the Invention
The preferred embodiment of the present invention is related to a method for modifying a photomask layout, and more particularly, to a method for estimating the number of dissections of an edge on a photomask layout before performing OPC (optical proximity correction).
2. Description of the Prior Art
In a photolithographic stage, an optical proximity effect often occurs, which jeopardizes the performance of a semiconductor device when patterns on the mask are transferred onto the wafer surface. Optical proximity effect causes deviation of patterns transferred on the wafer. Such deviations are usually related to the characteristics of the patterns to be transferred, the topology of the wafer, the source of the light, and various process parameters.
Optical proximity correction is one of the methods to correct and compensate for the deviations caused by the optical proximity effect. The commercially available OPC software can correct the original photomask layout using a theoretical image, so as to obtain correctly exposed photomask image layouts on the wafers.
The key point of the OPC is the dissection of the edges on an original photomask layout. In the traditional process, the manufacturer chooses hot points on the original photomask layout, and empirically estimates the number of segments of an edge on the hot point. Then, operational parameters including the number of segments are sent to the OPC software to obtain a corrected photomask layout. The above-mentioned original photomask layout refers to the desired photomask layout on the wafer after the photolithographic step. The hot point refers to regions at which the optical proximity effect often occurs. The hot point may be, for example, at the pitch or the bridge.
Although selecting many hot points can generate a more precise corrected photomask layout, much time is wasted in calculating the unneeded hot points. If, however, a hot point is not selected by the manufacturer based on his experience, the corrected photomask layout generated by the OPC software may become rough.
Therefore, a rule for selecting hot points and estimating dissecting segments is needed to replace the selection procedure based on human experience.